Previous experiments have revealed that nuclei of eukaryotic cells contain an enzyme which polymerises the ADP-ribose moiety of NAD through a 1'-2'-ribose-ribose linkage to form poly(ADP-ribose). The polymer is at least in part associated with basic chromosomal proteins and in particular histone H1. The current objective is to characterize the nuclear proteins to which poly(ADP-ribose) is attached with special emphasis on histone H1 and to elucidate the significance of this protein-poly(ADP-ribose) association in so far as nuclear processes during cellular growth is concerned.